under 26 free at Paris museums

Museum discounts for age are normally for EU citizens only. All but a couple or three countries issue national identity cars which are used for such purposes (citizens of the other countries use their passports). It would seem that showing a drivers license would be unusual since it's not what's commonly used for identity purposes as it is in the United States. I've no idea if a German license shows nationality. I don't even know if it shows a residental address. It doesn't really matter what it shows, actually. He'd be trying to obtain something fraudulently.

As an example, the Louvre website says that their free admission for under 26s policy is based on residency in the EEA rather than nationality. Under that he would probably qualify, but that may not be the case everywhere.

Members of the U.S.Forces are not residents of Germany. They fall under the Status of Forces Agreement (SOFA) and enjoy many privileges, e.g. no taxes and tax free shopping.
No European country accepts a drivers licence for identification and every country in Europe issues Identity Cards. Inge

Now that I'm awake, here's a visual, non-European example: I have a Chinese driver's license since it makes one little chunk of my life easier. If my silly, anglo-saxon looking self walked up to vote, odds are it wouldn't cut the mustard (not that there aren't silly anglo-saxon looking Chinese citizens).

There are no identity cards in Denmark, Ireland, Norway (although they may have changed this recently), and the United Kingdom within the EEA. It is very common to use driving licences as ID in the UK - at least recent ones with a photo. It certainly is the case that visiting armed forces are not usually considered to be ordinarily resident in the strict legal sense, and have significant exemptions - this does not mean that they would be necessarily refused such a concession (when I was in charge of this on a local matter I specifically included them as I was acting as a bridge into the local community). I guess you can only ask as you can't be in a worse position.

Thank you for your quick replies.